In conventional telephone exchanges of the type in use at the present time, line circuit functions must be provided for each line to provide control and supervisory functions for that line. In addition, each line which is to be part of a key sub-system usually includes an added key control circuit or line card to provide the signalling and hold functions for that line.
These line card circuits usually include a plurality of relays to control ringing and lamp control, ring cut-off and hold control and signalling. In addition, the line card circuits include elements for sensing a hold condition to switch a holding bridge across the line to perform the holding of the line during the pendency of the hold condition.
Some line card circuits known in the art employ electronic components in place of relays to provide these control functions.